MnB steels are standardized in EN 10083-3. these steels are highly hardenable and allow a reliably regime during hot pressing that makes it possible economically to bring about martensite hardening while still in the mold in the course of hot forming, without additional cooling. A typical example of a steel of this kind is that known under the designation 22MnB5, which can be found in the 2004 German steel codex (Stahlschlüssel) under material number 1.5528.
Fully killed 22MnB5 steel available on the market, as well as containing iron and unavoidable impurities, typically contains (in wt %) 0.10-0.250% C, 1.0-1.4% Mn, 0.35-0.4% Si, up to 0.03% P, up to 0.01% S, up to 0.040% Al, up to 0.15% Ti, up to 0.1% Nb, in total up to 0.5% Cr+Mo, and also up to 0.005% B.
For hot-rolled MnB steel sheets provided with an Al coating and intended for production of steel components by hot press hardening, EP 0 971 044 B1 specifies an alloying protocol whereby an MnB steel, in addition to iron and unavoidable impurities, is to have (in wt %) a carbon content of more than 0.20% but less than 0.5%, a manganese content of more than 0.5% but less than 3%, a silicon content of more than 0.1% but less than 0.5%, a chromium content of more than 0.01% but less than 1%, a titanium content of less than 0.2%, an aluminum content of less than 0.1%, a phosphorus content of less than 0.1%, a sulfur content of less 0.05%, and a boron content of more than 0.0005% but less than 0.08%. The Al coating is what is called an AlSi coating, consisting of 9-10 wt % Si, 2-3.5 wt % iron and aluminum as the balance. The coated flat steel products of this nature are heated to a heating temperature of more than 700° C., then inserted into a press mold, formed therein while hot to give the steel component, and at the same time cooled at a rate such that hardened microstructure is developed in the steel substrate of the flat steel product.
In the as-supplied state, all grades of manganese-boron steels are low in hydrogen. The amounts of diffusible hydrogen therein are in each case below the detection limit of currently 0.1 ppm. As a consequence of this, MnB steels display in principle only a low propensity toward belated, hydrogen-induced cracking.
In practice, however, it has emerged that in the hot forming of manganese-boron steels coated with aluminum-based protective coatings, hydrogen accumulates in the steel substrate under relatively moist oven atmospheres. The reason identified for this is a reaction between metal and water vapor. This reaction occurs when the flat steel product for the hot forming, bearing the Al coating, is heated to relatively high temperatures in a heating oven in an atmosphere containing water vapor. In this environment, the water vapor present in the oven atmosphere reacts at the surface of the material to form hydrogen and a metal oxide. The hydrogen formed diffuses into the steel material, where it may result in delayed failure, by concentrating preferentially in regions of high intrinsic tensile stress. If a locally very high hydrogen concentration is reached, this weakens the binding of the grain boundaries of the steel substrate microstructure to an extent such that in use, as a result of the stress that occurs, there is cracking along the grain boundary.
In order to avoid the introduction of hydrogen as a consequence of the surface reaction in the oven, dew point regulators are frequently used. The objective here is to limit the supply of water vapor in the oven atmosphere.
One example of such an approach is the method described in EP 1 767 286 A1. With this method, a flat steel product bearing an Al or Zn coating is hot-formed by heating to a temperature which is not less than the Ac3 temperature and not more than 1100° C. under a dry atmosphere whose hydrogen content is not more than 6 vol %, preferably not more than 1 vol %, and whose dew point is maintained at not more than 10° C. While the steel substrate of the flat steel product thus heated consists preferably of an MnB steel with 0.05-0.5 wt % C, 0.5-3 wt % Mn, and up to 0.05 wt % B, there may be 0.1-1 wt % Cr, 0.5-10 wt % Mg, 0.1-1 wt % Ti or 1-5 wt % Sn in its Al and Zn coating for the purpose of improving the corrosion control. The Al coating in this case is preferably an AlSi coating with 3-15 wt % Si. Iron may be present as an impurity in the Al coating, in amounts of typically 0.05-0.5 wt %. None of the working examples presented in EP 1 767 286 A1, however, is concerned with an AlSi coating which as well as the unavoidable Fe impurities includes an additional alloy constituent.
Particularly in regions where high atmospheric humidity is the rule, the effort and complexity entailed in providing sufficient quantities of dry air or dry nitrogen give rise to considerable operating costs.